1. Field of the Invention
The present invention relates to a metallic structure with plasmon resonance absorption and a photodetector.
2. Description of the Related Art
A fine metallic body (e.g., a metallic microparticle having a nanometer size) may exhibit an optical response called “local (surface) plasmon resonance absorption” in a specific wavelength region among broad wavelength regions ranging from the visible to infrared regions, depending on the shape or size of the metallic body. The examples of the metal exhibiting the local plasmon resonance absorption include noble metals, such as gold, silver, and platinum; however, even if the metal type is the same, if the size or shape of the metal differs, the local plasmon resonance absorption wavelength also differs. Attempts have been made to apply, to various optical devices, the nature that the absorption wavelength varies depending on a difference in size or shape of fine metallic bodies.
A metallic structure having a plurality of fine metallic bodies arranged in a substrate may have the plasmon resonance absorption in a broad region ranging from the visible to the infrared region, based on the principle of the local (surface) plasmon. In attempting to apply such metallic structure to an optical device or a sensor, it is important to adjust the wavelength region of this plasmon resonance absorption.
Moreover, for example, as shown in “Applied Physics A, vol. 29, pp. 71-75 (1982),” a phenomenon in which infrared absorption is enhanced by an optical electric field enhancement phenomenon via the plasmon at the surface of a metallic structure has been found. However, the mechanism thereof has not been clarified yet, and a quantitative measurement method or the like of the enhanced absorption has not been established yet, either. Therefore, if a metallic structure having a plasmon resonance frequency in a desired infrared region can be fabricated, an optical device or measurement system using the above-described phenomenon can be constructed.
The wavelength region of the plasmon resonance absorption which the above-described metallic structure has is affected by the “slenderness (the aspect ratio if the fine metallic body is rod-shaped)” of the fine metallic body. In other words, if a slender (the aspect ratio is high) fine metal is used, the wavelength region of the plasmon resonance absorption shifts to the long wavelength side, while if a short (the aspect ratio is low) fine metal is used, the wavelength region of the plasmon resonance absorption shifts to the short wavelength side.
On the other hand, for example, as shown in “J. Phys. Chem. B, 108, 13066 (2004)” and “JACS, 125, 13915 (2003),” techniques for chemically bonding and linking a plurality of rod-shaped nano metallic (gold: Au) bodies are known. The nano metallic bodies linked by a chemical bond (e.g., streptavidin-biotin interaction) are coupled via a chemical substance, which is not a metal, and strictly speaking, the nano metallic bodies are not directly linked to each other.
As described above, in order to obtain a metallic structure having the plasmon resonance absorption on the long wavelength side (e.g., infrared region), it is necessary to form a slender fine metallic body (e.g., a rod-shaped fine metallic body having a high aspect ratio). However, a single slender fine metallic body would cause a multi-mode absorption to deteriorate the wavelength selectivity, causing a problem in use in an optical device or the like. Then, it is contemplated that a metallic structure linking the nano metallic bodies is formed to improve the wavelength selectivity.
However, even with the metallic structure made by linking the nano metallic bodies, the absorption peak width of plasmon resonance absorption is not become very sharp. In order to increase the wavelength resolution and improve the wavelength selectivity further, the absorption peak width is expected to be sharpened further.
On the other hand, a photodetector has not been achieved yet that uses a metallic structure having the resonance wavelength of local plasmon on the long wavelength side while not exhibiting a multi-mode absorption (i.e., wavelength selectivity is high), and that achieve the detection of light in the terahertz region.